1. Field of the Invention
The present invention relates to an oxide catalyst composition. More particularly, the present invention is concerned with an oxide catalyst composition for use in producing methacrolein or a mixture of methacrolein and methacrylic acid by reacting at least one member selected from the group consisting of isobutylene and t-butanol with a molecular oxygen-containing gas, wherein the oxide catalyst composition comprises, in specific ratios, molybdenum or a mixture of molybdenum and tungsten; bismuth; iron; antimony; at least one member selected from the group consisting of yttrium and the elements of the lanthanoid series exclusive of promethium; and at least one member selected from the group consisting of potassium, rubidium and cesium; and cobalt solely, or a mixture of cobalt and at least one member selected from the group consisting of magnesium and nickel.
The oxide catalyst composition of the present invention exhibits not only a prolonged catalyst life due to its excellent properties with respect to thermal stability and reduction resistance, but also excellent selectivity for the desired product. By the use of the oxide catalyst composition of the present invention for producing methacrolein or a mixture of methacrolein and methacrylic acid, it becomes possible to stably produce the desired product for a long time while holding down the amount of by-produced impurities, e.g. diacetyl. The produced methacrolein or mixture of methacrolein and methacrylic acid has low contents of the by-produced impurities, e.g. diacetyl, and such methacrolein or mixture of methacrolein and methacrylic acid is very advantageous as a raw material for producing methyl methacrylate having excellent transparency. A methyl methacrylate polymer having excellent transparency, which can be obtained by polymerizing such highly transparent methyl methacrylate monomer, can be advantageously used as a substitute for glass and quartz in application fields requiring high transparency, such as optical fibers, light guide plates and the like; thus, such highly transparent methyl methacrylate polymer has very high commercial value.
2. Prior Art
A polymer produced from methyl methacrylate is characterized in that it is glassy, hard and transparent, and such polymer is frequently used as a substitute for glass. In recent years, in the fields related to optical fibers, a methyl methacrylate polymer is attracting attention as an optical material which can substitute for quartz, and the use of the methyl methacrylate polymer is spreading. Therefore, the methyl methacrylate polymer used as a substitute for glass or quartz is required to have high transparency and high weathering resistance. For obtaining such an excellent methyl methacrylate polymer, it is important that, in a methyl methacrylate monomer used as the raw material therefor, the amounts of trace impurities be very small which lower the transparency and weathering resistance of the methyl methacrylate polymer.
As methods for producing methyl methacrylate, which is a compound highly useful in the industry, there are known two methods: a “direct ML-to-MMA process” comprising two reaction steps and a “via methacrylic acid process” comprising three reaction steps. The direct ML-to-MMA process comprises two catalytic reaction steps, wherein the first reaction step comprises subjecting isobutylene and/or t-butanol as a starting material to a gaseous phase catalytic oxidation reaction with a molecular oxygen-containing gas in the presence of an oxide catalyst (hereinafter, this catalyst is frequently referred to as a “first stage catalyst”) to thereby obtain methacrolein, and the second reaction step comprises subjecting the obtained methacrolein to a gaseous phase catalytic reaction with methanol and a molecular oxygen-containing gas in the presence of a carrier-supported catalyst containing palladium (hereinafter, this catalyst is frequently referred to as a “second stage catalyst”), to thereby produce methyl methacrylate (MMA) by one step directly from methacrolein (ML).
In the recent studies by the present inventors on the direct ML-to-MMA process, it has been found that substances which show an absorption in the visible light range of 400 nm to 780 nm, are causative of discoloration of methyl methacrylate. The substances showing an absorption in the visible light range include not only diacetyl, which is conventionally known as being causative of discoloration, but also pyruvic aldehyde, 2-acetylfuran and the like. Thus, pyruvic aldehyde, 2-acetylfuran and the like have been found to be substances causative of the discoloration of methyl methacrylate.
The first stage catalyst used in the direct ML-to-MMA process (wherein the first stage catalyst is used for producing methacrolein by subjecting at least one member selected from the group consisting of isobutylene and t-butanol to a gaseous phase catalytic oxidation reaction with a molecular oxygen-containing gas) was proposed by the present inventors (see, for example, International Patent Application Publication No. WO 95/35273 and Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-216523). However, at the time of the development of the catalyst, it was not well recognized that the impurities, e.g. diacetyl, which are by-produced by the catalyst are causative of the discoloration of methyl methacrylate.
U.S. Pat. No. 4,249,019, Japanese Patent Application Publication No. Sho 57-35859, U.S. Pat. No. 4,518,796 and the like propose various types of palladium-containing second stage catalyst for use in the second reaction step of the direct ML-to-MMA process (wherein the second stage catalyst is used for producing methyl methacrylate by subjecting methacrolein to a gaseous phase catalytic reaction with methanol and a molecular oxygen-containing gas). In the studies of the present inventors, it has been found that, since the second stage catalytic reaction (for producing methyl methacrylate by reacting methacrolein, methanol and a molecular oxygen-containing gas) is conducted under moderate reaction conditions wherein the reaction temperature is in the range of from room temperature to 100° C., the impurities, e.g. diacetyl, which would be causative of the discoloration of the resultant methyl methacrylate remain almost unreacted and undecomposed during the second stage catalytic reaction, and the impurities as such are carried over into a subsequent purification step. When the impurities causative of discoloration are carried over into the purification step, it is necessary to perform repeatedly the purification operation for removing the impurities from the methyl methacrylate. As a result, great commercial disadvantages are posed in that the repeated purification operation causes a loss of methyl methacrylate, leading to an increase in the production cost.
The via methacrylic acid process is also a method for producing methyl methacrylate by using isobutylene and/or t-butanol as a starting material. The via methacrylic acid process is described at pages 172 to 176 of “Sekiyu Kagaku Purosesu (Petrochemical Process)”, published by Kodansha Scientific, Inc., Japan. The document states that the via methacrylic acid process comprises three reaction steps, that is, a first oxidation step, a second oxidation step, and an esterification step. The first oxidation step is a step of subjecting at least one starting material selected from the group consisting of isobutylene and t-butanol to a gaseous phase catalytic oxidation reaction with molecular oxygen in the presence of a catalyst, to thereby obtain methacrolein. The second oxidation step is a step of subjecting the methacrolein obtained in the first oxidation step to a gaseous phase catalytic oxidation reaction with molecular oxygen in the presence of a catalyst, to thereby obtain methacrylic acid. The esterification step is a step of subjecting the methacrylic acid obtained in the second oxidation step to esterification, to thereby obtain methyl methacrylate.
Various proposals have been made on the catalyst used in the first oxidation step of the via methacrylic acid process, that is, the catalyst used for producing methacrolein by subjecting at least one starting material selected from the group consisting of isobutylene and t-butanol to a gaseous phase catalytic oxidation reaction. Most of such proposals are concerned with the selection of the types and ratios of the components of the catalyst. For example, there can be mentioned Examined Japanese Patent Application Publication No. Sho 48-17253 (corresponding to Canadian Patent No. 947,772), U.S. Pat. Nos. 4,001,317 and 4,537,874, and Unexamined Japanese Patent Application Laid-Open Specification Nos. Sho 60-163830, Sho 63-122641 and Hei 2-227140. The catalysts disclosed in these patent documents are aimed mainly at achieving an improved yield of the desired product; and, in these patent documents, the experimental data concerning the performance of the catalysts are only the data of the conversion of isobutylene and t-butanol and the data of the yield of and selectivity for methacrolein or methacrylic acid.
With respect to the by-produced impurities formed in the via methacrylic acid process (that is, the products other than methacrolein and methacrylic acid which are, respectively, the desired products of the first and second oxidation steps of the via methacrylic acid process), the descriptions of prior art documents are as follows. For example, Examined Japanese Patent Application Publication No. Sho 53-23809 describes the selectivities for acetic acid, CO2 and CO; Examined Japanese Patent Application Publication No. Sho 57-61011 describes the selectivities for acetone and acetic acid; and Examined Japanese Patent Application Publication Nos. Sho 51-13125 and Sho 51-12605 each describe the selectivities for CO2 and CO. In addition, U.S. Pat. No. 3,928,462 (corresponding to Examined Japanese Patent Application Publication Nos. Sho 47-32043 and Sho 47-32044) describes that the selectivity for acrolein is 5% to 6%. All of the impurities described in the above-mentioned patent documents are different from the substances which are causative of discoloration.
In addition, Examined Japanese Patent Application Publication No. Hei 5-86939 describes that a gaseous, oxidation reaction product, which is obtained by subjecting at least one member selected from the group consisting of isobutylene and t-butanol to a gaseous phase catalytic oxidation for producing methacrolein, contains not only methacrolein and methacrylic acid, but also low boiling point by-products (such as acetoaldehyde, acetone, acrolein, acetic acid and acrylic acid), high boiling point by-products (such as maleic acid and aromatic carboxylic acids), polymeric substances and tarry substances. For obtaining a gaseous, oxidation reaction product containing substantially no polymeric substances and the like, the above-mentioned patent document proposes a method in which the gaseous, oxidation reaction product is contacted with a solid alkaline earth metal compound to thereby suppress the formation of the polymeric substances and the like, and also proposes a method in which the polymeric substances and the like contained in the gaseous, oxidation reaction product are decomposed and removed therefrom. This patent document describes the amounts of the by-produced maleic acid and the by-produced polymeric substances, in addition to the amounts of the produced methacrolein and the produced methacrylic acid. However, this patent document has no description about the trace impurities which are causative of the discoloration of methyl methacrylate.
In addition, it is considered that, since a high reaction temperature, namely 300° C. to 400° C., is used in the second oxidation step of the via methacrylic acid process, most of the substances (such as diacetyl) causative of discoloration which are by-produced in the first oxidation step are decomposed during the second oxidation step of the via methacrylic acid process. Consequently, the substances causative of discoloration have not been particularly considered as being a problem in the case of the via methacrylic acid process. However, since not all of the substances causative of discoloration are decomposed in the second oxidation step of the via methacrylic acid process, it is necessary that a catalyst which does not by-produce the substances causative of discoloration be used in the first oxidation step of the via methacrylic acid process.
In connection with the first and second oxidation steps of the via methacrylic acid process, Japanese Patent No. 2509049 (corresponding to U.S. Pat. No. 5,264,627) describes a measure for reducing the amounts of the impurities which are causative of the discoloration of methyl methacrylate. In this patent document, the via methacrylic acid process is performed as follows. At least one compound selected from the group consisting of isobutylene, t-butanol and methyl-t-butyl ether is introduced, together with molecular oxygen, into a shell-and-tube heat exchanger type first oxidation reactor packed with an oxide catalyst containing bismuth, molybdenum and iron, and a gaseous phase catalytic oxidation reaction is effected therein to thereby obtain a gaseous reaction product comprised mainly of methacrolein. Subsequently, the methacrolein-containing gaseous reaction product and molecular oxygen are introduced into a shell-and-tube heat exchanger type second reactor packed with an oxide catalyst containing molybdenum and phosphorus, and a gaseous phase catalytic oxidation reaction is effected therein to thereby produce a gaseous reaction product comprised mainly of methacrylic acid. In this method, the space of the gas outlet portion of the second reactor is packed with a solid packing so as to reduce the volume of the space (of the gas outlet portion) which is present downstream of the catalyst bed in the second reactor, wherein the reduction of the volume of the space of the gas outlet portion is intended to shorten the residence time of the gaseous reaction product in the space of the gas outlet portion, thereby suppressing the by-production of diketones. This patent document further states that, when diketones are contained in methacrylic acid obtained after the first and second oxidation steps, a problem arises in that the diketones are converted into furan compounds in the final methacrylate polymer and the furan compounds cause discoloration of the polymer. (In the Working Examples of this patent document, acetonitrile acetone is mentioned as a diketone.)
Therefore, when the catalyst used in the first reactor used in the above-described method is improved so as to decrease the amount of the by-produced diketones, the improved catalyst is effective for decreasing the amounts of the substances causative of the discoloration of methyl methacrylate produced by the via methacrylic acid process.
As can be seen from the descriptions of the above-mentioned patent documents, it has been recognized to some extent that the discoloration of methyl methacrylate produced by the via methacrylic acid process is caused by the impurities, e.g. diacetyl, which are by-produced in the first oxidation step of the via methacrylic acid process, namely by-produced in a reaction in which at least one member selected from the group consisting of isobutylene and t-butanol is subjected to a gaseous phase catalytic oxidation reaction in the presence of a catalyst, thereby producing methacrolein. However, there have been no methods for improving the catalyst used for producing methacrolein in the first oxidation step of the via methacrylic acid process wherein the improvement is for decreasing the amounts of the substances (such as diacetyl) causative of discoloration which are by-produced in the first oxidation step of the via methacrylic acid process.
When methyl methacrylate is produced by the direct ML-to-MMA process, oxidative methyl esterification (the second stage reaction) is conducted at a low temperature (room temperature to 100° C.). The advantage of the direct ML-to-MMA process is that the yield of methyl methacrylate is higher than in the case of the via methacrylic acid process. However, the direct ML-to-MMA process has a problem in that most of the substances (such as diacetyl) causative of discoloration are not decomposed by the second stage reaction catalyst and are carried over into the subsequent purification step. Therefore, in the direct ML-to-MMA process, for improving the quality of methyl methacrylate, it is necessary to decrease the amount of the substances causative of discoloration, which are impurities by-produced in the first stage reaction. Thus, there is a strong demand for the development of a catalyst which is advantageous not only in that a high selectivity for methacrolein can be achieved, but also in that the catalyst has high thermal stability and high reduction resistance, and the selectivity for the impurities causative of discoloration can be held down to a minimum level.